In a leisure vehicle such as a motorcycle, cylinders of an engine are typically arranged to extend substantially vertically, namely in a substantially upright position. Especially in a multi-cylinder engine of the leisure vehicle, the cylinders extending substantially vertically are arranged in-line transversely, i.e., in a width direction of a vehicle body of the vehicle (see Japanese Laid-Open Patent Application Publication No. Hei. 5-79347). Such a transverse-mounted in-line engine is able to advantageously reduce a mechanical loss, because a drive system such as a valve system has a simple construction, in contrast to a V-type engine in which cylinders are arranged in V-shape. Nonetheless, when a transverse-mounted in-line engine is equipped in a motorcycle, the longitudinal direction of the cylinders is in the vertical direction, and a projected area of the engine as viewed from the front, that is, a frontal-projected area of the engine, increases, and as a result air resistance increases.
In a transverse-mounted in-line engine that employs a water cooling system, a space between a radiator disposed forward of the engine and a front end of the engine becomes small. To extend an exhaust pipe from an opening end of an exhaust port of the engine that opens forward, it is necessary to curve the exhaust pipe from forward to downward and from downward to rearward with small curvature radiuses. In this case, a pressure loss and hence a fluid resistance of the exhaust pipe increase, decreasing exhaust efficiency. In addition, cooling efficiency of the radiator is likely to decrease because the radiator is subjected to heat radiated from the exhaust pipe located behind the radiator. If the exhaust pipe or the engine (cylinder, etc.) is positioned behind and near the radiator, an amount of cooling air flow passing through the radiator from forward to rearward decreases, decreasing cooling efficiency of the radiator.
Since the cylinders are arranged to extend substantially vertically in the above transverse-mounted in-line engine, a vertical dimension of the engine increases, and thus, a vertical space in an area where the engine is disposed is limited.
In an engine including the cylinders arranged to extend substantially vertically, pistons vertically reciprocate, causing vertical vibration to occur in the engine. A rider riding on the vehicle feels the vertical vibration. To reduce the vertical vibration, some engines have a counterweight mounted to counteract the reciprocation of the piston. In such an engine, a weight of movable components increases, causing slow acceleration and deceleration. This is unfavorable to a high-speed engine which requires high-speed response.
In motorcycles, if the position of the engine is greatly changed to solve the above mentioned problem, then the center of gravity of the motorcycle is correspondingly changed. If the center of gravity of the engine is greatly changed, then braking characteristics or cornering characteristics of the motorcycle may change. For this reason, the ability to change the position of the engine is limited.